Welsh author Jo Walton's story of a 15-year-old girl with magical abilities at an English boarding school has beaten books from Stephen King and George RR Martin to win the British Fantasy award for best novel.

King's time-travelling tale of JFK's assassination, 11.22.63, was also up for the best novel prize, as was Martin's latest in the bestselling Song of Ice and Fire series, A Dance With Dragons. Walton's novel, which is also in the running for the World Fantasy award, tells the story of Morwenna, a teenager who lives with her mother in Wales, a woman who "spins dark magic for ill". When she is crippled in a magical battle, she leaves for an English boarding school where she buries herself in magic and in reading. Walton was born in Wales and now lives in Canada.

The award for best horror novel went to Adam Nevill's The Ritual, a tale of a group of friends on a walking holiday in Sweden who stumble across a dead creature hanging from a tree and fall into a world of ancient and evil pagan rituals. Lavie Tidhar's Gorel and the Pot-Bellied God won best novella; best collection went to Robert Shearman's Everyone's Just So So Special; and best anthology to Jeff and Ann VanderMeer's The Weird.

"The Weird collects over 850,000 words of weird fiction from 20 countries and the last 100 years," wrote Jeff VanderMeer. "As we said in our pre-recorded acceptance speech, since we could not attend, David Hartwell's Dark Descent and Alberto Manguel's Black Water were touchstones for us. The process of collecting and getting permissions for over 110 stories nearly killed us, but receiving the award helps heal the scar tissue."